[securedrop-export] combine usb-test and disk-test into a single disk-test

[redshiftzero]

Followup from freedomofpress/securedrop-export#41

We currently have two pre-check actions for before disk export (export to a removable block device, generally a USB): usb-test and disk-test. We could combine these into a single action to simplify the interface. Thoughts? This would involve coordinated changes in securedrop-client and securedrop-export - since it involves modifying the inter-VM interface.

[gonzalo-bulnes]

@rocodes We may also be addressing this issue as part of freedomofpress/securedrop-export#105 🙂 Flagging so that we can reference it in the PR is relevant.